Moving coil electrical instrument



March 22, 1960 Filed Aug. 16, 1956 'Figll.

J. C. NYCZ ETAL MOVING COIL ELECTRICAL INSTRUMENT 2 Sheets-Sheet l.

INVENTOR Joseph C. Nycz 8 Lawrence J. Lunos.

ATTORNEY 2 Sheets-Sheet 2 Fig. 6.

85 z m R OWN. H T N R VC 0 m m n 3 .A sr ow JOWZ L W J. c. NYCZ ETAL I MOVING con. ELECTRICAL INSTRUMENT Fig." 3.

March 22, 1960 Filed Aug. 16, 1956 Fig 5.

WITNESSES:

MOVING con. ELECTRICAL INSTRUMENT Joseph C. Nycz, Towaco, and Lawrence J. Lunas, Cedar Grove, N.J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of lennsylvania Application August 16, 1956, Serial No. 604,428

1 Claim. (Cl. 324-144) This invention relates to moving-coil electrical instruments and it has particular relation to moving-coil electrical measuring instruments having ferromagnetic structures defining air gaps for the associated moving coils.

Moving-coil instruments are employed in various fields, such as for measuring and relaying. The advantages of ferromagnetic structures for moving-coil instruments, particularly those of the electrodynamometer type, long have been recognized. The deterrents to adoption of such magnetic structures have been lack of accessibility and lack of accuracy. Ferromagnetic structures are structures constructed from a material, usually containing iron, which has a magnetic permeability substantially greater than that of a vacuum.

Recently moving-coil instruments of the electrodynamometer type have become available, which include ferromagnetic structures defining air gaps for the associated moving coils wherein accuracy is obtained without sacrifice of accessibility. Examples of such instruments will be found in United States Patents 2,438,027 and 2,508,- 410.

In the aforesaid patents, a moving-coil assembly is associated with a ferromagnetic structure which is divided into two spaced sections. These sections have passages permitting the installation and removal of the associated moving-coil assembly without disturbing the magnetic structure in any way. Each of the magnetic.

sections produces a net solenoid force acting on the coil assembly in response to energization of the coil assembly alone. These net solenoid forces are observed to be non-uniform so as to vary as the coil assembly movesin its path of travel. In order to reduce errors due to these solenoid forces the two sections are arranged such that the solenoid forces are oppositely directed.

It is observed that instruments of the type discussed above are still subject to a force responsive to energization of the moving-coil assembly alone which urges the moving-coil assembly towards a position intermediate the ends of the path of travel of the coil assembly, usually a midscale position. This force is quite small and for some applications of instruments may be neglected. However, it does introduce an error and the magnitude of the error is dependent on the energization of the moving coil. This force appears to be due to a variation in the self-inductance of the coil assembly as the coil assembly moves in its path of travel, and is the resultant of the above-mentioned oppositely directed,'non-uniform net solenoid forces. This force will be referred to herein as a force F.

In accordance with the invention, a moving-coil instrument of improved construction is provided having a magnetic structure defining an air gap for the moving-coil assembly with the structure designed to substantially eliminate the aforesaid force F. In a preferred embodiment of the invention the instrument includes two reversely arranged magnetic sections formed of a plurality of identical laminations configured to provide air gaps having length dimensions uniform throughout the 2,929,994 Fatented Mar. 22,

path of travel of the coil assembly. The laminations are further configured such that the resultant force F produced by the magnetic sections has a substantially zero value throughout the path of travel of the coil assembly.

It is, therefore, an object of the invention to provide an improved moving-coil instrument having a ferromagnetic structure defining one or more air gaps for the moving-coil assembly.

It is another object of the invention to provide an improved instrument as defined in the preceding paragraph wherein the ferromagnetic structure is configured'and arranged to facilitate installation and removal of the coil assembly relative to the structure.

It is a further object of the invention to provide an improved moving-coil instrument having a magnetic structure formed of a plurality of laminations configured and arranged to eliminate a torque developed in response to energization of the moving-coil assembly alone which urges the moving-coil assembly towards an intermediate position in its path of travel.

It is still another object of the invention to provide an instrument as defined in the preceding paragraph wherein the laminations are of identical configuration providing air gaps of uniform length throughout the path of travel of the moving-coil assembly.

Other objects of the invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure l is an exploded view in perspective with parts broken away of an electrical measuring instrument embodying the invention;

Fig. 2 is a view in perspective with parts broken away showing the instrument of Fig. 1 in assembled condition;

Fig. 3 is a view in top plan with parts broken away of the instrument shown in Figs. 1 and 2;

Fig. 4 is a view in side elevation of a two-element electrical measuring instrument;

Fig. 5 is a graphical representation of the solenoid forces or torques developed by the instrument of Fig. 1; and

Fig. 6 is a view in top plan of a magnetic lamination of different configuration than laminations employed in the instrument of Fig. 1.

Referring to the drawings, Fig.1 shows an electrodynamometer instrument which includes a stator assembly 1 and a rotor assembly 1a.

The rotor assembly comprises a shaft 9 which is mounted for rotation by suitable bearing screws 11, 13 which are part of the stator assembly. The shaft 9 supports a coil assembly 3 preferably comprising two series connected coils 4 and 5 lying in a common plane carried by a suitable holder 12 secured to the shaft. The coil 4 has coil sides 6 and 7 and the coil 5 has coil sides 8 and 10 with the coil sides 6, 7, 8 and 10 parallel to the shaft 9.

A pair of spiral, flexible, electroconductive strips 15, 17 have their inner ends secured to insulating bushings 19, 21 which are carried by the shaft. The outer ends of the strips are connected to lugs 23, 25 for the purpose of establishing connections between the movable coils 4 and 5 and an external circuit through conductors 27 29. The terminals of the coils 4 and 5 are connected to the inner ends of the spiral strips 15, 17, respectively. For damping purposes the shaft 9 carries an electroconductive' disk 35 which is mounted for rotation between the poles of a permanent magnet 37. A control spring 31 has its inner end connected to the shaft 9, and its outer end con nected to a lever 33 adjustably secured to thestator a ssembly. The control spring biases the rotor assembly towards a predetermined angular position. Indicating means such as an arm 39 is attached to the shaft.

arm carries a pen 41 across the surface of a chart 43'. The chart may be advanced continuously relative to the pen 41 in a manner well understood in the art.

The stator asesmbly 1 includes a magnetic structure 45 which establishes magnetic paths for the magnetic fluxes produced by currents flowing in the coils 4 and and in fixed windings 4'7 and 49 which are associated with the magnetic structure. The structure 45 alone provides magnetic paths for flux produced by currents in the coils 4 and 5 and the windings 47 and 49 inasmuch as the rotor assembly includes no magnetic parts. It is noted further that these magnetic paths are confinedto an area located between a pair ofspaced parallel planes which extend transverse to the coil sides 6 and to intersect these coil sides.

The magnetic structure 45 includes a magnetic section 51 having a substantially continuous magnetic body or rim portion 53 which substantially surrounds the shaft 9 and the coils 4 and 5. A pair of pole pieces 55 and 57 (Fig. 2) project towards each other from opposite interior surfaces of the rim portion 53 along a straight line 58 (Fig. 3) defining an axis of symmetry of the rim portion 53 to providearcuate pole faces adjacent respectively the paths of travel of the coil sides 6 and 10.

In addition the magnetic section 51 has a pair of cantilever arms or magnetic cores 59 and 61 which project from the remaining opposite interior faces of the rim portion 53 to intersect the line 58 to pass respectively through the coils 4 and 5 on opposite sides of the shaft 9. These magnetic cores 59 and 61 have inner surfaces or faces 59a and 61:: which are spaced in a direction transverse to the shaft 9 by a distance sufiicient to permit passage of the movable coils 4 and 5 therebetween. Furthermore, the magnetic cores 59 and 61 have outer arcuate surfaces or faces spaced from the pole pieces 55 and 57 to provide a pair of arcuate air gaps within which the coil sides 6 and 10 are disposed for movement.

It will be noted with reference to Fig. 3 that the magnetic cores 5-9 and 61 provide a substantially cylindrical magnetic core which is attached on opposite sides to the rim portion 53 and which has the passage 63 extending therethrough. Since the passage 63 communicates with the air gaps in which the coil sides 6 and 10 are positioned, the coil assembly 3 may be rotated in a counterclockwise direction (as viewed looking at the rotor assembly from the control spring end) to bring the coil assembly into alignment with the passage 63. The coil assembly then may be moved in a direction parallel to the shaft 9 through the passage 63.

The magnetic section 51 may be formed of any suitable soft magnetic material having a magnetic permeability substantially greater than that of a vacuum. Such materials are termed ferromagnetic or generally magnetic materials, an example being silicon iron. Preferably, a material having low hysteresis loss is employed. The magnetic section 51 may be formed of a solid piece of soft iron. However, it is preferable to form the magnetic section 51 of a plurality of laminations 64 as shown in Figs. 1 and 2, particularly if the instrument is designed for measuring alternating-current quantities. The laminations may be provided with suitable openings 65 through which rivets may be passed for the purpose of securing the laminations together. If the magnetic section is formed of laminations, the desired contour of each lamination may be accurately formed by a punching operation.

By inspection of Figs. 1 and 2, it will be noted that a separate magnetic path is provided for each of the coil sides 6 and 10. The magnetic path for the coil side 6 includes the pole piece 55 and the magnetic core 59 together with the air gap therebetween. The winding 47, when energized, directs magnetic flux through this magnetic path to provide a magnetic field for the coil side 6. In a similar manner, the magnetic path for the coil side 10 includes the magnetic core 61 and the pole piece 4 57 together with the air gap therebetween. When the winding 49 is energized, magnetic flux is directed through the associated magnetic path to establish a magnetic field for the coil side 10.

Although the magnetic section 51 alone may be employed, an improvement in performance may be obtained by adding thereto an additional magnetic section 67. The additonal magnetic section serves to reduce errors resulting from the solenoid action of the section 51 as will be explained hereinafter.

The magnetic section 67 is similar in construction to the magnetic section 51 but is reversed with respect to the magnetic section 51 about a line transverse to the shaft 9 for a reason appearing hereinafter. The magnetic section 67 is preferably formed of a plurality of laminations 68 having a pair of magnetic cores 69 and 71 which extend respectively through the coils 4 and 5 on opposite sides of the shaft 9. In addition, the magnetic section 67 has a pair of pole pieces 73 and 75 which are positioned respectively in the windings 47 and49. It will be observed that the magnetic cores 69 and 71 have inner surfaces or faces 69a and 71a which are spaced to provide a passage 77 therebetween which corresponds to the passage 63 of the magnetic section 51.

By inspection of Fig. 1, it will be observed that the passages 63 and 77 are displaced angularly about the shaft 9 with respect to each other. Consequently, the coil assembly 3 cannot be removed from the magnetic structure 45 by a simple movement thereof in the direction of the shaft 9. To permit removal of the coil assembly from the magnetic structure the magnetic sections 51 and 67 are spaced from each other along the shaft 9 by a distance sufiicient to permit movement of sides of the coil assembly 3 therebetween. The desired spacing may be provided by any suitable spacer formed of either magnetic or nonmagnetic material.

In the embodiment illustrated in Figs. 1 and 2, the spacer is divided into two parts 79 and 79a. Each of the parts is in the form of a pluralityof magnetic laminations which are similar in construction to the adjacent parts of the laminations of the magnetic section 51. In order to facilitate inspection of the space between the magnetic sections 51 and 67 after assembly thereof, the parts 79 and 79a are located at a substantial distance from each other to provide an opening 7% (Fig. 2) in the magnetic structure 45. The space between the magnetic sections is clearly visible through this opening. Consequently, when the magnetic sections 51 and 67 are assembled with the spacer therebetween as shown in Fig. 2, a space is provided between the pair of magnetic cores 59 and 69 and the pair of cores 61 and 71. This space is sufficient to permit movement of a side of the coil assembly 3 therebetween which is transverse to the sides 6 and 10.

It is believed that the operations required to assemble and disassemble the instrument illustrated in Figs. 1 and 2 are apparent from the foregoing discussion. To facilitate a further description of such operations, reference will be made to a leading'side 4a of the coil 4 and a leading side 5a of the coil 5 (the lower side of the coil assembly 3 as viewed in Figs. 1 and 2), and a trailing side 4b of the coil 4 and a trailing side 5b of the coil 5 (the upper side of the coil assembly as viewed in Figs. 1 and 2). It will be understood that the magnetic structure 45 comprising the laminations of the magnetic sections 51 and 67 and the laminations of the spacer is first completely assembled as shown in Fig. 2 wherein a rivet 45a is disclosed for uniting the laminations to each other. Also the rotor assembly 1a is completely assembled, the complete assembly including the shaft 9, the coil assembly 3, the conductor strips 15 and 17, the disk 35, the arm 39 and the control spring 31. The rotor assembly then is placed above the magnetic structure 45 (as viewed in Figs. 1 and 2) with the leading side 4a of the coil 4 and the leading side 5a of the coil 5 aligned with the passage 63 of the magnetic section 51. The rotor assembly including the coil assembly 3 is then lowered in a direction parallel to the axis 9 to pass the leading sides of the coils 4 and 5 completely through the passage 63. The leading sides of the coils 4 and 5 are now positioned between the pair of magnetic cores 59 and 69 and the pair of cores 61 and 71.

To complete the insertion of the coil assembly 3 into operative position, the rotor assembly including the coil assembly next is rotated in a'clockwise direction (looking at the rotor assembly from the control-spring end thereof) to bring the leading sides of the coils into alignment with the passage 77 of the magnetic section 67. During such rotation of the coil assembly 3 the leading sides move between the magnetic sections 51 and 67. After the leading sides have been brought into alignment with the passage 77, the rotor assembly is lowered in a direction parallel to the shaft 9 to pass the leading sides completely through the passage 77. The coil assembly now is positioned to embrace the complete resultant magnetic core formed by the magnetic cores 59, 61, 69 and 71. The bearing screw 11 and the support therefor are next placed in position, and the bearings screws 11 and 13 are adjusted to mount the rotor assembly for rotation with respect to the magnetic structure. The outer ends of the conductor strips 15 and 17 are soldered to the lugs 23 and 25 and the permanent magnet 37 is positioned as shown in Fig. 1. To complete the installation of the rotor assembly, the outer end of the control spring 31 is soldered or otherwise secured to the lever 33. By following a reverse procedure the rotor assembly 1a may be removed from the magnetic structure 45 Without disturbing the magnetic structure in any Way.

From the foregoing discussion, it is clear that the magnetic structure 4-5 is formed of a plurality of unitary laminations each of which has integral pole pieces and magnetic cores. Because of this construction the magnetic structure may be provided with accurate air gaps, and the accuracy of the air gaps is not disturbed by assembly or disassembly of the instrument.

In certainvapplications a two-element electrodynamic instrument is required. Such an instrument may be constructed in the manner illustrated in Fig. 4. Referring to Fig. 4, a two-element electrodynamic instrument is disclosed which includes the two elements 81 and 83. The element 81 comprises a magnetic structure 85 which is similar in construction to the magnetic structure 45 of Figs. 1 and 2. It will be observed that the magnetic structure 85 has associated therewith a pair of fixed windings 87 and 89 which correspond to the fixed windings 47 and 49 of Figs. 1 and 2. In addition, the magnetic structure 85 has disposed therein a movable coil assembly 91 comprising two coils 92 and 94 which corresponds to the movable coil assembly 3 of Figs. 1 and 2.

The element 83 is similar in construction to the element 81 and includes a magnetic structure 93, fixed windings 95 and 97 and a movable coil assembly 99 comprising two coils 98 and 100. The magnetic structures 85 and 93 are mounted on suitable supporting posts 101 and are spaced from each other sufficiently to permit rotation of one of the movable coil assemblies therebetween. In certain cases it may be desirable to place magnetic shields 102 between the fixed windings 87 and 95 and between the fixed windings 89 and 97 to prevent magnetic interference between the windings on opposite sides of the shields.

The movable coil assemblies 91 and 99 are secured to a common shaft 103 for rotation therewith. This shaft carries a pair of conductor strips 105 for connecting the terminals of the movable coil assembly 99 to an external circuit and a pair of conductor strips l07 for connecting the terminals of the movable coil assembly 91 to an external circuit. These conductor strips correspond to the conductor strips 15 and 17 of Fig. 1. In addition, the disk 35, the arm 39 (which may support an indicating pointer or a recording pen), and control spring 31 S are secured to the shaft 103 with the disk 35 positioned for movement between the poles of the permanent magnet 37. As well understood'in the art,'each of the elements 81 and 83 may be energized from a separate pair of conductors of a three-wire circuit or from a separate phase of a polyphase circuit.

Since the principles employed in the construction of the instrument illustrated in Figs. 1 and 2 are also employed for the instrument of Fig. 4, it follows that the rotor assembly in Fig. 4 may be introduced into operative position with respect to the magnetic structures .85 and 93 or may be removed therefrom without disturbing the magnetic structures in any way. For example, in

constructing the instrument, the magnetic structures 85 and 93 are completed and are secured to the supporting posts 101. For convenience in discussing the assembly of the instrument, the coils 98 and 100 will be referred to as having respectively leading sides 98a and 100a and trailing sides 98b and 10%. The coils 92 and 94 will be referred to as having respectively leading sides 92a and 94a and trailing sides 92b and 94b. This corresponds to the notation employed for the coil assembly 3 of Figs. 1 and 2.

The rotor assembly of Fig. 4 is first completely assembled. This rotor assembly includes the shaft 103, the coil assemblies 91 and 99, the conductor strips 105 and 107, the disk 35, the pen arm 39 and the control spring 31. The rotor assembly then is placed above the magnetic structure as viewed in Fig. 4 with the leading sides 98a and a of the coil assembly 99 positioned above the adjacent passage in the magnetic structure 85. the rotor assembly then is dropped in a direction parallel to the shaft 103 rotated and again dropped to position the coil assembly 99 for embracing the magnetic cores of the magnetic structure 85. This procedure is exactly similar to that employed for dropping the coil assembly 3 of Figs. 1 and 2 through the passages 63 and 77 to embrace the associated magnetic cores.

The coil assembly 99 then is passed completely through the magnetic structure 85 by rotating the coil assembly 99 until its trailing sides 98b and 10Gb are in position to drop through the adjacent passage in the magnetic structure 85. After the trailing sides have passed through the adjacent passage, the coil assembly 99 is rotated to pass the trailing sides 98b and 1001; between the magnetic sections of the magnetic structure 85 until the trailing sides are positioned to drop through the lower passage in the magnetic structure. The coil assembly 99 now is lowered to a position between the magnetic structures 85 and 93.

The operation of passing a coil assembly completely through its magnetic structure may be understood more fully by a further consideration of Figs. 1 and 2. Assuming that the coil assembly 3 is in the position illustrated in Figs. 1 and 2 and that it is desired to drop the coil assembly completely through its associated structure 45, the coil assembly is rotated until its trailing sides 4b and 5b are adjacent the passage 63 in the magnetic section 51. The coil assembly-now is lowered until the trailing sides are positioned between the magnetic sections 51 and 67. By suitably rotating the coil assembly 3 in a clockwise direction (looking at the rotor assembly from the control spring end thereof) the trailing sides are moved through the magnetic sections 51 and 67 to a position wherein the trailing sides are in alignment with the passa'ge 77in the magnetic section 67. Thetrailing sides now maybe dropped through the passage 77 to completebly 99 into alignment with the adjacent passage of the magnetic structure 93. The coil assembly 99 next is dropped, rotated and again dropped in the manner previously'discussed with reference to the coil assembly 3 of Figs. 1 and 2 until the coil assembly )9 is in position to embrace the magnetic cores of the associated magnetic structure 93. Since the magnetic structures 85 and 93 are similar, the movement of the coil assembly 99 from a position between the magnetic structures 85 and 93 to a position wherein the coil assembly 99 embraces the magnetic cores of the magnetic structure 93 also moves the coil assembly 91 from a position above the magnetic structure 85 to a position wherein the coil assembly 91 embraces the magnetic cores of the associated magnetic structure 85. Consequently, both of the coil assemblies 91 and 99 are in their operative positions with respect to their associated magnetic structures. With the rotor assembly of Fig. 4 in this position, the hearings associated with the shaft 103 may be adjusted and the outer ends of the conductor strips 105 and 107 may be connected as discussed with reference to Figs. 1 and 2. In addition, the outer end of the control spring 31 may be connected to its associated lever 33 and the permanent magnet 37 may be moved to operative position with respect to the disk 35. The instrument of Fig. 4 now is in completely assembled condition. By following a reverse procedure, the rotor assembly of Fig. 4 may be removed completely from the magnetic structures 85 and 93 without disturbing the magnetic structures in any way.

Referring again to Figs. 1 and 2, the windings 47 and 49 are connected to series and are so energized that if direct current is passed therethrough, magnetic flux flows through the pole pieces in the directions illustrated by the arrows 109 and 110 of Fig. 1. If the windings are energized by alternating current, the arrows 109 and 110 represent instantaneous directions of magnetic flux flow. In order to provide a magnetically astatic construction, the energization of one of the windings may be reversed. Such astatic construction also is discussed in the aforesaid Patent 2,508,410.

With the exception of the configuration of the magnetic laminations 6 and 68 the instruments illustrated in Figs. 1, 2, 3 and 4 are exactly similar to instruments illustrated and described in the aforesaid Patent 2,508,410. The reversely related sections 51 and 67 shown in Figs. 16 and 17 of Patent 2,508,410 reduce to a substantial extent the value of the force resulting from solenoid action produced in response to energization of the moving coil assembly alone from the value which would exist if only the section 51 were employed. However, a force still exists which tends to move the moving coil assembly towards a midscale position. It will be helpful at this time to review briefly the present understanding of the theory underlying the force acting on the moving coil assembly.

Referring to Fig. 3, it will be noted that when the windings 47 and 49 are deenergized and the moving-coil assembly 3 is energized, magnetic flux ispoduced which passes through the coil 4 and crosses the air gap between the core 59 and the pole piece 55. This magnetic flux is illustrated in Fig. 3 by dotted lines 111. It will be understood that magnetic flux crosses each of the air gaps of the sections in substantially the same manner. Let it be assumed for present purposes that the laminations 64 of the section 51 are replaced by laminations each having the configuration of the lamination L illustrated in Fig. 6. As there shown, the configuration of the lamination L differs from that of one of the laminations 54 in the shaping of the arms forming the magnetic cores. It is noted that corresponding portions of the laminations L and 64 have the same reference numeral but that the numerals for the lamination L have the prefix L- With the assumed arrangement the section 51 of in Figs. 1 and 2 is identical to the section 51 of Figs. 16 and 17 of Patent 2,508,410.

For this assumption then the magnetic section 51-is asymmetric with respect to the path of travel of the movable-coil assembly 3. When the coil assembly 3 is in its extreme counterclockwise position as viewed in Fig. 3, the magnetic reluctance of the magnetic path in the section 51 offered to magnetic flux produced by current flowing through the coil assembly 3 is a maximum. Consequently, the self-inductance of the coil assembly is a minimum for this condition. Conversely, when the coil assembly 3 is adjacent its extreme clockwise position the magnetic reluctance offered to magnetic flux produced by current flowing in the coil assembly 3 is a minimum. For this condition the self-inductance of the coil assembly is a maximum. Consequently, when the coil assembly 3 is energized and the windings 47 and 49 are deenergized, the coil assembly 3 tends to take a position wherein the magnetic reluctance of the associated magnetic path is a minimum, or wherein the self-inductance of the coil assembly is a maximum. This may be termed a solenoid action and the net force applied to the coil assembly 3 by the solenoid action of the section 51 urges the coil assembly in a clockwise direction. It has been observed that this net force is non-uniform so as to decrease as the coil assembly 3 is urged in a clockwise direction as viewed in Fig. 3.

Insome cases, as when the energization of the fixed windings is constant, it is possible to calibrate the instrument to read correctly despite the presence of this solenoid action. However, the value of the net force due to this solenoid action may be reduced from the value present when only the section 51 is employed by the provision of the additional magnetic section 67. The compensation permits more correct indication by the instrument for substantially all applications thereof. The effect of the reversely related sections 5?. and 67 is to provide two oppositely directed net solenoid forces. However, since these net forces are non-uniform they do not cancel entirely but provide a resultant non-uniform force which urges the moving-coil assembly towards a midscale position. This resultant force will be termed a force F. For some applications, the error introduced by the force F may be neglected. However, it is desirable that this source of error be eliminated insofar as possible. It is believed that the force F is due to a variation in the self-inductance of the coil assembly over the path of travel of the coil assembly.

The problem may be considered further with reference to Fig. 5, wherein ordinates represent torque applied to the moving-coil assembly 3 of Fig. 3 and abscissae represent the angle of displacement of the moving-coil assembly 3 from its zero position which isassumed to be the counterclockwise end of the path of travel of the moving-coil assembly as viewed in Fig. 3. Under the assumption that all laminations of the magnetic sections 51 and 67 are similar to the lamination L of Fig. 6, the previously mentioned net solenoid forces developed by the magnetic sections 51 and 67, when the coil assembly 3 is energized by a predetermined current, may be represented in Fig. 5 by curves 1-15 and 116. These net solenoid forces are assumed to be oppositely directed and non-uniform throughout the angular movement of the coil assembly. Consequently, these net solenoid forces do not entirely compensate each other but provide the resultant force F represented by the curve F1 in Fig. 5 which urges the moving-coil assembly towards a midscale position (45 in Fig. 5). It is observed that the force F has a positive value acting in a clockwise direction when the coil assembly occupies its extreme counterclockwise position, and a negative value acting in a counterclockwise direction when the coil assembly occupies its extreme clockwise position. It is assumed in Fig. 5 that the coil assembly is capable of movement through an angle of The curve F1 also shows that in the midscale position of the coil assembly (45" no force F is applied to the coil assembly.

In accordance with the present invention the force F is substantially eliminated by an improved configura-- tion of the magnetic sections 51 and 61 effective to produce two net solenoid forces which provide a resultant force having a substantially zero value throughout the path of travel of the coil assembly. In a preferred embodiment of the invention the inner surfaces 59a, 61a, 69a and 71a of the magnetic cores are shaped to produce the desired result. 7

Such shaping may conveniently be obtained by suitable punching of laminations having the configuration of the lamination L. By inspection of the magnetic of the inner faces L59a and L6la of the laminations L which lie between the axis 58 and the tips of the arms L59 and L61 to form the laminations 64 having oppositely extending pockets 59b and 61b each located at a separate side of the axis 58. It has been observed that good results may also be obtained by configurations of the-pockets 59b and 61bother than the configurations illustrated, or by removal of material from the inner faces at locations other than the locations illustrated. It i will be understood that the magnetic section 67 is exactly similar to the magnetic section 51 except for the reversal thereof about a line transverse to the shaft 9.

The effect of the tapering of the magnetic cores of the sections 51 and 67 in the manner described is to provide two net solenoid forces which are uniform and equal throughout the path of travel of the movingecoil assembly 3 about its axis of rotation. For example, the curve 119 in Fig. 5 may represent the net solenoid force developed by the magnetic section 51 formed of the laminations 64, whereas the curve 121 may represent the net solenoid force developed by the magnetic section 67 formed of the laminations 68. These net solenoid forces are observed to be uniform throughout the path of travel of the coil assembly in any plane which extends transverse to the shaft 9 to intersect the air gaps. The sum of the two curves 119 and 121 is represented by the curve 118 which is observed to coincide with a curve representing zero torque. Consequently, by suitable shaping of the magnetic cores it is observed that zero resultant force is applied to the coil assembly 3 throughout its path of travel when the coil assembly alone is energized. The forces represented in Fig. 5 are for a predetermined current flowing through the coil assembly 3. Inasmuch as all of these forces vary '10 in magnitude in accordance with variations in magnitude of the coil assembly current, the compensation ,is effective for all energizations of the coil assembly.

It is observed that a fully compensated instrument is provided wherein all the magnetic laminations are of identical configuration. This aspect of the invention facilitates the tooling and stockingof parts. Furthermore, by shaping only the inner faces of the magnetic cores to provide compensation air gaps having uniform length dimensions throughout the, path of travel of the coil assembly may be provided if desired. The uniform air gaps assist in providing a desirable linear scale distribution.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications are possible, and it is desired to cover all modifications falling within the spirit and scope of the invention.

We claim as our invention:

In an electrodynamic instrument, a pair of magnetic sections each formed, entirely of a plurality of identical magnetic laminations, each section having a closed rim portion with a pair of pole pieces extending from two opposed inner sides of the rim portion towards each other along a first line, each section having further a pair of cores extending from the two remaining opposed inner sides of the rim portion to intersect the vfirst line for providing a resultant cylindrical core, each of said cores having outer and inner faces terminating in a core tip, the outer faces of the cores of each section defining with the pole pieces of each section two air gaps spaced angularly about an axis transverse to the first line, means mounting the sections reversed relative to each other about a second line transverse to the axis with the air gaps of the two sections in alignment, a coil assembly having a pair of coil sides each disposed in a separate pair of aligned air gaps, and means mounting the coil assembly for rotation about said axis, the air gaps of the sections having length dimensions which are uniform throughout the path of travel of the coil assembly, the inner faces of the cores of each section being spaced to provide a passage through which the coil assembly may be introduced into operative position and removed from the sections without disturbing the sections, the inner face of each core having a concave face portion located between said first line and the tip of the core to provide net solenoid forces acting between the sections and the coil assembly in response to current flowing through the coil assembly alone which are substantially uniform and equal throughout the, path of travel of the coil assembly.

References Cited in the file of this patent UNITED STATES PATENTS 2,773,240 Young Dec. 4, 1956 

